The present invention relates to a high pressure sodium vapour lamp with improved color reproduction index, which in an outer bell or envelope contains a ceramic discharge device having at least a pair of electrodes as well as a pair of current supply lines for the current supply of the electrodes, wherein the outer bell contains a filling comprising sodium introducing exclusively in the form of an amalgam and a ignition gas (see IES Lighting Handbook, 1981, pages 8-42). The arrangement according to the invention is especially suitable for improving the color reproduction index of high pressure sodium vapour lamps of low (from 16 to 100 w) output.
In projecting high pressure sodium vapour lamps of higher color reproduction as well as one lying close to incandescent lamps, a color temperature of about 2500K represents the theoretical possibilities of improvements of the parameters which is known to specialists in the field: the temperature of the end region (the so-called cold chamber) of the discharge vessel must be increased and thereby the vapour pressure of the sodium can reach a higher value as well, than at lower temperatures. The increase of the vapour pressure of the sodium brings about a widening of the spectrum of the generated light, which leads to an improvement of the color reproduction. In the case of high pressure sodium vapour lamps of about 100 w and having outputs higher, the necessary increase of the temperature of the cold chamber required for the improvement of the color reproduction index is brought about, in first instance, in such a manner, that at the end of the discharge vessel metallic cradles are arranged, which are made from a metal sheet (for example, from niobium), which reduce the infrared emission tendency of the end regions. Such solution is, for example, found in the lamp of the type LU250/DX of the General Electric Corp. The important disadvantage of such type of solution resides in the slow warming up, as well as in the condition that the necessary increase in the temperature of the cold chamber can be achieved in a given manner only at lamp outputs exceeding 150 w. Under such output the lamps will exhibit a too large passive heat radiating upper surface with respect to the input power and, therefore, to attain the desired higher temperature is impossible, or to attain it, a very high wall loading of the discharge vessel would be necessary, which would lead to unresolvable problems and, particularly, to a shortening of the operating life, as well as to a constant reduction of the light output of the light source. For similar reasons a warming up of the discharge vessel for an extended period of time, involving the above-mentioned lamp, becomes a characteristic, which, especially in interior applictions, in many times, is inadmissible. The increase of the upper surface loading of the discharge vessel gives, therefore, especially in the case of high pressure sodium vapour lamps of low outputs and small dimensions, no guidance which could be followed in an endeavor to improve the color reproduction index in order to increase the color temperature.
In the article "50 w Super High-Pressure Sodium Lamp" by Y. Ogata et al (Illum. Eng. Inst. Japan, Conference 1983, Paper No. 61) a ceramic discharge vessel of special shape is illustrated for attaining the above object. The special shaping means a reduction of the ceramic pipe portion at both of the electrodes, whereby the temperature of the cold chamber can be increased. Such shaping makes possible the attaining of the set object according to experiments, however, the manufacturing of such narrowed down ceramic pipe presents a difficult technicological problem, which results in increased manufacturing costs.